1. Field of the Invention
The invention relates to a fixing device and an image forming apparatus having such a fixing device. The invention specifically relates to a fixing device that employs a non-thermal fixing system capable of fixing toner using a foam fixer.
2. Description of the Related Art
Image forming apparatuses such as a printer, a facsimile machine, a photocopier, and a multifunctional peripheral having these functions are generally configured to form images including characters or symbols on recording media such as paper, cloth, and OHP sheets based on image information. In particular, electrophotographic image forming apparatuses are widely used for forming high definition images on plain paper at high speeds. The electrophotographic image forming apparatus generally employs a thermal fixing system capable of fixing images at high fixing speeds with high quality. In the thermal fixing system, toner residing on a recording medium is melted, and pressure is applied to the melted toner on the medium, thereby fixing the toner on the recording medium.
However, in the electrophotographic image forming apparatus having the thermal fixing system, more than half of the power appears to be consumed for heating toner. This may lead the difficulty in improving an energy-savings in the electrophotographic image forming apparatus. In view of recent environmental concerns, the development of a low power consumption (energy-saving) fixing device has attracted much attention. That is, a fixing device having a non-thermal fixing system (hereinafter also called a “non-thermal fixing device”) that is capable of fixing toner on a recording medium without heating has been desired.
An example of such a non-thermal fixing device is disclosed in Japanese Patent No. 3290513 (hereinafter called “Patent Document 1”). Patent Document 1 discloses a wet fixing system as an example of the non-thermal fixing system, in which a surface of an object on which an unfixed toner is arranged at a predetermined position is sprayed or droplets are applied with an O/W emulsion fixer composed of an insoluble organic compound dispersed in water and capable of dissolving or swelling toner; after the application of the O/W emulsion fixer on the unfixed toner on the object to dissolve or swell the toner, the object having the dissolved or swollen toner is dried.
In addition, Japanese Patent Application Publication No. 2007-219105 discloses (hereinafter called “Patent Document 2”) another example for the non-thermal fixing system, in which a foam fixer formed by dispersing air bubbles in a liquid fixer is applied on a toner image formed on a recording medium. With this foam fixer fixing system, since the fixer is foam, the density of the fixer is lowered. Thus, a film thickness of the (foam) fixer on an application roller may be increased with a smaller amount of the fixer compared to the film thickness of the related art liquid fixer. As a result, it may be possible to reduce an adverse effect of liquid surface tension on the toner particles on the recording medium. Further, in this configuration, since the small amount of the fixer is applied on the toner on the recording medium, residual liquid on the recording medium may be appropriately suppressed, and image degradation due to particle flow caused by the fixer may also prevented. Thus, the toner image may be fixed without degradation using the amount of the fixer smaller than the related art fixer if the toner image is fixed with the foam fixer as described in Patent Document 2.
However, in the fixing device having such a foam fixer fixing system, it may be difficult to cause a foam application mechanism to generate a foam fixer (hereinafter also simply called “foam”) onto a sheet of paper in accordance with the sheet transferred to a foam application position in the right place at the right time. One of the reasons for the above may be the difficulty in maintaining the foam fixer formed of the mixture of air and the fixer for a long time. In addition, it may also difficult to generate foam fixer having foam films and foam density suitable for fixing the toner, including the control of the suitable foam films and foam density of the foam fixer. That is, at least the following conditions need to be controlled in forming foam fixer.
1. Foam generation needs to be controlled at a position of a front end of a first sheet.
2. Foam generation needs to be controlled between the continuously transferred sheets; that is, the foam generation needs to be controlled at a position of a rear end of the first sheet and at a position of a front end of a second sheet when sheets are continuously transferred.
3. Foam generation needs to be controlled at a rear end of a last sheet.
However, if the foam needs to be applied over an entire surface of the sheet, more controls need to be satisfied such as foam generation time (time to stabilize foam) or sheet registration variability.
In order to relax the above control conditions, that is, foam generation time, foam generation time between the sheets, and foam generation time after passing through the last sheet, a method for continuously generating foam may be given as an example. However, with this method, the amount of consumed foam may be increased, and hence foam generation may be stopped immediately after the last sheet is passed through the foam application position in order to decrease the amount of consumed foam. It is difficult to control the foam generation time between the sheets if a sheet transferring speed is high. Accordingly, it appears to be easier to control foam generation by the method of continuously generating a foam fixer in order to maintain accuracy and control of the foam generation time. However, continuously generating the foam may induce accumulation of the generated foam around a front end (entrance) of a nip portion where the fixer is applied to the recording medium that has been transferred to the nip portion.
When the foam residing on a foam application roller is applied to and pushed into the recording medium, the foam applied to the recording medium may be pressed with the pressure of a pressure roller. However, if the sheet is not located inside the nip portion, the generated foam that has passed through the nip portion is pushed out from the entrance of the nip portion with the pressure of the pressure roller, thereby accumulating the foam around the nip entrance. The foam is accumulated around the nip entrance because neither the foam application roller nor the pressure roller has a configuration to suction the foam.
Such accumulation of foam around the nip entrance may prevent the recording medium from entering the nip portion, may cause paper jamming, or may attach to both sides of the sheet. Further, the surface of the sheet may have non-uniform foam application. As a result, too much application of the foam may be obtained at the front end of the sheet, thereby degrading sheet separation.